In a typical mixer-settler, in the first step, the aqueous and organic phases are pumped into a mixer or mixers in order to achieve a uniform liquid-liquid dispersion and a small droplet size. After mixing, the dispersion is fed into a settler. The settler is typically a large tank which is square in plan and its square area is about several hundred square meters. Dispersion is fed into the settler at the feed end of the settler. A distributor fence is arranged adjacent to the feed end of the settler to distribute the flow of the dispersion to the whole width of the settler. In the settler, the dispersion moves towards the settler discharge end and, at the same time, the phases separate by gravity into two layers with a dispersion band remaining between them.
The settler tank is normally built on the site. WO 2007/135221 A1 discloses one method for manufacturing a mixer-settler on the site. Wall structures are connected by vertical support columns to the bottom plate. The wall structure is formed by fastening a required number of horizontal support beams to the vertical support columns at regular intervals. A required number of plate-like wall elements made of a chemically resistant material are attached to the horizontal support beams inside the mixer-settler, so that they form a load-bearing structure in the spaces left between the horizontal support beams. The plate-like wall elements are connected to the plate-like element covering the bottom plate of the mixer-settler. However, as mentioned, such a settler is still a large tank which is square in plan and its square area is about several hundred square meters.
A conventional solvent extraction settler has some disadvantages. In a large settler tank, transverse flow patterns may exist, slowing down the coalescence in the dispersion. The specific area of the wall surfaces is small, whereby the advantageous wall effect is minimal and the coalescence is slow. Maintenance of the settler requires shutting down the whole solvent extraction process because it is not possible to perform maintenance operations, e.g. for the removal of accumulated crud, while the process is running. Further, the capacity of the settler cannot be increased easily. The process cannot be run with only a part of its capacity. The atmosphere above the liquid surface in the settler is flammable because it contains volatile organic compounds which are released from the hydrocarbon based solvents. Fire protection in a conventional settler is also difficult because the fire compartment in which the fire may ignite has an area which is as large as that of the whole settler. If leakage occurs in the settler tank, the amount of damages can be very large.